This invention relates to a brake-slip-controlled brake system suited for automotive vehicles, in particular for road vehicles, which brake system is provided with a master cylinder acted upon by the pedal force in a direct or auxiliary-force-assisted manner and connected with the wheel brake cylinders via hydraulic or pneumatic brake circuits, which system further has transducers and a braking pressure modulator which, in dependence on the braking behavior of the vehicle, generates pulsations which are reducing the braking pressure.
In the known brake-slip-controlled brake systems the rotational behavior of the brake-slip-controlled wheel e.g. is permanently measured by an inductive sensor and compared with the vehicle's velocity or with a corresponding reference value. If the values measured or the changes, respectively, reveal a lock-up tendency the braking pressure will be reduced in a steady or pulsating manner. As soon as the controlled wheel is re-accelerating the braking pressure will be built up anew so as to enable the wheel to contribute to the braking operation.
The braking behavior of the individual wheels and of the entire vehicle is determined by a plurality of factors largely varying within wide limits. Factors of influence are the road conditions in dependence on the weather and the state of the road, the adhesive power of the tires, the static and dynamic axle loads, as well as the axle load distribution, the rating and the state of the brake system, etc. Trouble factors and inaccuracies in detecting and processing of the values measured are likewise important. For these reasons and because of the high safety and reliability demands which are to be made on principle with regard to brake systems brake-slip-controlled brake systems are relatively expensive and intricate. Thus there exist numerous treatises and publications with regard to the relationships between the signals measured and suitable measures of control and regulation which may be realized at reasonable expense as well as with regard to the selection and design of sensors, evaluation circuits, and braking pressure modulators.
It is also known to limit control to particular wheels only or to one axle alone in order to simplify a slip-controlled brake system and to reduce the manufacturing expense. In such arrangements, the other wheels are acted upon by the same braking pressure or by the braking pressure derived from the controlled wheels or by uncontrolled braking pressure (European Patent Application No. 51,801). Thus, under certain circumstances it will be impossible to avoid the lock-up of particular wheels. Depending on the system chosen or on the limitations chosen as compared with brake systems whose control acts on all wheels, one will accept a loss in the steerability or an increase in the stopping distance in certain situations which will be relatively rare.
In order to avoid a control-device-caused increase in the stopping distance in driving situations to be mastered with particular difficulties such as may occur when driving in deep snow, sand, on rocky stones, or on roads with a relatively thin coating of ice it is further already known to suppress control at one wheel at predetermined time intervals so as to cause said wheel to lock (DE-OS=German Published Patent Application No. 22 58 317).
Further, anti-skid control systems have been described which have pulsators in order to improve the driving stability and the steerability of the vehicle by intermittent braking. (DE-PS=German Pat. No. 20 48 802, DE-AS=German Examined and Printed Patent Application No. 23 34 493). In these systems, controllable single-stage or two-stage pumps are used as pulsators. When the difference between the wheel's rotational deacceleration and the vehicle's deacceleration exceeds a certain value, the pumps are switched on so as to periodically modulate the braking pressure at the respective wheel. In a known system (DE-AS=German Examined and Printed Patent Application No. 23 34 493) the amplitude of the pressure reduction or of the pulsation, respectively, depends on the braking pressure adapted and supplied by way of the brake pedal which may be achieved by means of a control sleeve determining the relieving volume, the control sleeve being adjusted in dependence on the brake pedal force. In this arrangement, a sensor is used as a detecting element for the switching-on of the pulsator. The sensor operates on the principle of the variable response threshold. By means of the sensor, the rotational deacceleration of the wheel(s) is compared with the deacceleration of the vehicle. Control systems of this type are very expensive. No additional complicated measures being provided, control systems of said type are disadvantageous in that the amplitude of the braking pressure, which is pulsating in case of control, will substantially only depend on the braking pressure adapted and supplied by way of the brake pedal.
It is thus an object of this invention to overcome the described disadvantages of the prior art systems and to provide a brake-slip-controlled brake system which will be characterized by a comparatively simple structure and modest manufacturing expense, yet will ensure that in all situations occurring in practice the driving stability and the steerability of the vehicle will be maintained during the braking action, with a stopping distance being achieved which will be as short as possible. Great store was likewise set by a low consumption of energy during control as this will have favorable effects on the costs of manufacture.